tracing: Update trace-event-sample with TRACE_SYSTEM_VAR documentation

Add documentation about TRACE_SYSTEM needing to be alpha-numeric or with
underscores, and that if it is not, then the use of TRACE_SYSTEM_VAR is
required to make something that is.

An example of this is shown in samples/trace_events/trace-events-sample.h

Link: http://lkml.kernel.org/r/20150403013802.220157513@goodmis.orgReviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

samples/trace_events/trace-events-sample.h

@@ -22,7 +22,25 @@
  * protection, just like TRACE_INCLUDE_FILE.
  */
 #undef TRACE_SYSTEM
-#define TRACE_SYSTEM sample
+#define TRACE_SYSTEM sample-trace
+
+/*
+ * TRACE_SYSTEM is expected to be a C valid variable (alpha-numeric
+ * and underscore), although it may start with numbers. If for some
+ * reason it is not, you need to add the following lines:
+ */
+#undef TRACE_SYSTEM_VAR
+#define TRACE_SYSTEM_VAR sample_trace
+/*
+ * But the above is only needed if TRACE_SYSTEM is not alpha-numeric
+ * and underscored. By default, TRACE_SYSTEM_VAR will be equal to
+ * TRACE_SYSTEM. As TRACE_SYSTEM_VAR must be alpha-numeric, if
+ * TRACE_SYSTEM is not, then TRACE_SYSTEM_VAR must be defined with
+ * only alpha-numeric and underscores.
+ *
+ * The TRACE_SYSTEM_VAR is only used internally and not visible to
+ * user space.
+ */
 
 /*
  * Notice that this file is not protected like a normal header.